In the manufacture of printed wiring boards, such as boards having an epoxy coated metal substrate, an inked circuit configuration is applied to a surface of the substrate; the ink acting as a catalyst for accelerating the adherence of copper onto the substrate's surface during a later step in the manufacturing process. One of the methods for applying an inked circuit configuration onto a substrate surface involves a screen process using a silk screen having thin screen areas, called land areas or pads, representing the circuit configuration to be applied to the substrate's surface, and thick screen areas surrounding the land areas. In this process, the screen is placed over the surface of the printed wiring board substrate, ink is poured on the screen, and a squeegee is then moved over the screened surface forcing the ink through the thin screened, or land, areas onto the printed wiring board surface. When the screen is removed from the surface, an inked circuit configuration conforming to the land areas of the silk screen remains on the substrate's surface.
Often printed circuit board substrates have thru holes drilled in the substrate to allow for electrical connection between circuits appearing on the top and bottom surfaces of the substrate. In order for good electrical connections to be formed in the holes during a later step in the manufacturing process, the walls of each hole must be coated with ink. The above silk screening method, however, does not allow for the inking of the walls or sides of thru holes which appear in the printed wiring board substrate. To ink the walls of thru holes, the printed wiring board typically undergoes another step in the manufacturing process called "ball printing," which step is normally applied prior to the silk screening step.
Ball printing involves the use of a plate having pins extending vertically from the plate. Each pin is located on the plate so that it is aligned with a corresponding thru hole in the printed wiring board substrate. The diameter of each pin is slightly (a few mils) greater than the diameter of its corresponding thru hole.
The heads of the pins are dunked into a bath of ink and the plate is then properly aligned over the substrate with each inked pin located directly over its corresponding thru hole. The plate is lowered until the ink covered head of each pin rests in each hole. Since the pin diameter is only slightly greater than the hole diameter, the pin rests in the crater or lip of the hole and does not enter the hole. The plate is removed, leaving a bubble, or ball, of ink covering each hole and, by applying a vacuum to the bottom of the printed wiring board substrate, the ink is then sucked down through and covers the walls of the hole.
Ball printing, as described above, involves an additional step in the manufacturing process of the printed wiring board and thus adds time and expense to the process, resulting in a more costly printed wiring board.
Additionally, it is frequently required to have an electrical circuit connection extend around the edge of a printed wiring board substrate for interconnecting the circuits appearing on each side of the substrate. "Ball printing," as described above, will not apply ink to the edges of the substrate and the edge is presently coated with ink by an operator hand brushing the ink onto the edge.